Often, it is necessary in an industrial or other process to inject a measured quantity of a flowable material into a further stream of material or a vessel. Metering pumps have been developed for this purpose and may be either electrically or hydraulically actuated. Conventionally, an electromagnetic metering pump utilizes a linear solenoid which is provided half-wave or full-wave rectified pulses to move a diaphragm mechanically linked to an armature of the solenoid. FIGS. 1 and 2 illustrate a conventional control strategy for an electromagnetic metering pump pumping against ten bar and five bar force levels, respectively. In the conventional electromagnetic metering pump, the solenoid is electrically powered at a sufficient level to provide a pumping force at maximum air gap (i.e., zero stroke) which will meet or exceed the maximum pumping force expected to be encountered. The electric power is also delivered at maximum power level at all other stroke positions, resulting in a wasting of force and energy and development of heat. The heat that is generated typically results in the need for components that can tolerate same, such as metal enclosures and other metal parts and/or larger solenoids with more copper windings. In addition, the extra forces applied to the armature result in the need for relatively heaver return springs and components to counteract residual magnetism and allow the armature to return in time for the pump diaphragm to do suction work. Still further, sound levels are increased owing to the banging of the armature at the end of the stroke when pumping against lower force levels, and further due to the striking of the armature against a stroke adjustment stop at the end of each suction stroke under the influence of the heavy return spring. Service life is typically short owing to the mechanical stresses that are encountered.
In an effort to overcome these problems, a different control methodology has been implemented which has been graphically illustrated in FIGS. 3 and 4. In FIG. 3, the solenoid is energized by a pulse train consisting of full-wave rectified sine waves followed by half waves. This control methodology allows the pump to be more efficient, thereby permitting larger capacity models to be completely housed in corrosion resistant plastic owing to the lower levels of heat that are produced. FIG. 4 illustrates yet another modification wherein the ratio of half-wave to full-wave pulses is adjustable so that a user can reduce power if lower pressures are encountered. One can see by an inspection of FIGS. 3 and 4 that wasted force and energy (and thus heat) are reduced as compared with the conventional technology illustrated in FIGS. 1 and 2. However, even with these significant advancements in control methodology, it would be desirable to further reduce the wasting of force and energy in the operation of the pump.